Process for the utilization of vanadium bound in chromium ore as vanadium(V) oxide by electrolysis

ABSTRACT

Process for recovering the vanadium present in the chromium ore chromite, as vanadium pentoxide, as a by-product from the vanadium-containing sodium chromate solution produced in the process of preparing sodium chromate and sodium dichromate from said ore, by precipitation of calcium vanadate from the sodium chromate solution, treatment of the precipitate with water and sodium carbonate, sodium bicarbonate, carbon dioxide or sodium carbonate and carbon dioxide, to precipitate calcium carbonate and electrolysis of the solution remaining after separation of the calcium carbonate precipitate, to produce vanadium pentoxide.

[0001] The invention relates to a process in which the vanadium present in the chromium ore chromite is recovered as vanadium pentoxide during the course of the fusion of the chromium ore with alkali and its work-up to produce sodium chromate solution and the important chromium chemical sodium dichromate.

BACKGROUND OF THE INVENTION

[0002] All processes used for producing sodium dichromate Na₂Cr₂O₇-2H₂O via sodium chromate solution employ a procedure having the same principle:

[0003] Chromium spinel or chromite is mixed with sodium carbonate and/or sodium hydroxide and iron oxide (recovered ore) and heated at 1000-1100° C. in the presence of oxygen. The sodium chromate produced is leached from the resulting reaction mixture by means of water at a controlled pH. During this procedure, the vanadium present in the chromite also goes into solution as sodium vanadate. Control of the pH is necessary to suppress the dissolution of iron, aluminium, silicon and magnesium. In general, addition of an acid, e.g. a dichromate solution, is necessary to adjust the pH. After leaching with water is complete, the sodium chromate solution produced is converted into a sodium dichromate solution by addition of sulphuric acid or preferably of carbon dioxide under pressure. Solid sodium dichromate is recovered from the solution by evaporation and crystallization. This process is described in BÜCHNER, SCHLIEBS, WINTER, BÜCHEL “Industrielle Anorganische Chemie”, Weinheim 1984, and in Ullmann's Encyclopedia of Industrial Chemistry, Fifth ed., Vol A 7, Weinheim 1986, p. 67-97.

[0004] It has now been found that the vanadium content of the sodium dichromate (about 0.2% of V₂O₅ in the Na₂Cr₂O₇.2H₂O) interferes in various applications of the sodium dichromate and its downstream products, so that purification of the sodium chromate fusion solution to remove the vanadium before conversion into sodium dichromate is made desirable.

[0005] The removal of vanadium is preferably carried out by addition of calcium oxide to the sodium chromate solution produced by leaching and filtration to remove insoluble material. The solution is in this way brought to a pH of 12-13 (EP-A-0 047 799, EP 0 453 913 B1), resulting in precipitation of a filterable calcium hydroxyvanadate Ca₅(OH)(VO₄)₃. The removal of the calcium which has been introduced in excess is carried out by subsequent precipitation of calcium carbonate from the sodium chromate solution, as described in EP 0 453 913 B1.

[0006] A consequence of the precipitation of calcium hydroxyvanadate from a solution having a high concentration of chromate ions is the high contamination of the calcium hydroxyvanadate by coprecipitated calcium chromate and sodium chromate and by entrained calcium oxide. The V₂O₅ content of the dried “calcium vanadate” precipitate is about 10-20% and is thus significantly below the V₂O₅ content of pure calcium hydroxyvanadate Ca₅(VO₄)₃OH of 48.7%, but also significantly higher than the V₂O₅ content of naturally occurring vanadium-containing ore of not more than 2.4% of V₂O₅ (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5^(th) ed., Vol. A27, page. 370). The “calcium vanadate” precipitate, having a V₂O₅ content of 10-20% is thus an attractive starting material for producing vanadium oxide, due to its high vanadium content as well as its high reactivity which results from its finely divided nature and low crystallinity. In addition, the disposal of such a material as waste in a landfill is not acceptable because chromate and vanadate would be released to the environment over time, and the material therefore would have to be made inert by treatment with reducing agents such as iron(III)sulphate or sulphur dioxide or the like before it could be disposed of in a landfill.

[0007] In the processing of this calcium vanadate precipitate, its valuable constituents, namely vanadium as V in the oxidation state +5 and chromium as Cr in the oxidation state +6, have to be obtained as separate or easily separable species, the vanadium in particular in a readily usable and commercial form and the chromium in a utilizable form, e.g. one that is able to be reintroduced into the chromate production process, i.e. recyclable, species or solution. The readily soluble sodium, which is undesirable in the wastewater, is to be recovered as a utilizable, recyclable species and the calcium as an insoluble species which can be disposed of in a landfill or as precursor of a reusable calcium oxide or reusable calcium chromate or calcium dichromate.

[0008] Solutions containing sodium ions and chromium in the oxidation state +6 to be reintroduced into the chromate production process can be so utilized only if they do not contain appreciable amounts (e.g. >0.1%) of interfering extraneous material. However, any elements and oxidation states which are not already present to an appreciable extent in the product stream into which this solution is to be introduced will interfere. For this reason, only water H₂O, hydroxide OH⁻ or hydroxonium H₃O⁺ ions, carbonate, bicarbonate, carbon dioxide, chromate, dichromate, polychromate, chromic acid, sodium and also calcium and vanadate in very minor concentrations are permissible as constituents of the solutions which are to be reintroduced into the chromate production process. Depending on the pH, these solutions can be introduced into the acidification steps of the sodium chromate process (e.g. after or during leaching of the furnace clinker) or the alkalization steps (e.g. before or during vanadate precipitation).

[0009] The digestion of the calcium vanadate precipitate by means of sulphuric acid, separation of the precipitated calcium sulphate from the solution in accordance with a treatment of insoluble calcium salts which is frequently practiced in industry and subsequent precipitation of the vanadium as V₂O₅ from the filtrate by means of sulphuric acid is known.

[0010] However, a consequence is that the chromium is obtained as a polychromate or chromic acid solution having a high sulphate content. As the many proposals for removing sulphate from sodium chromate and dichromate demonstrate, the amounts of sulphate always introduced as sulphuric acid in earlier processes for producing sodium chromate and sodium dichromate are nowadays totally undesirable for further processing of these products (EP 0 453 913 B1).

[0011] Another way of treating sparingly soluble salts is digestion of the calcium vanadate precipitate by means of sodium carbonate in aqueous solution and subsequent precipitation of the vanadium as ammonium metavanadate (NH₄)₄V₄O₁₂, also referred to as NH₄VO₃ for short, by addition of an excess of ammonium salts. Ammonium metavanadate is a versatile intermediate, in particular in the route to the most important vanadium chemical V₂O₅ (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5^(th) edition, Vol. A27, Weinheim 1996). However, in this case too, the resulting solution containing the chromium as chromate is contaminated with the ions of the precipitation reagent, i.e. with ammonium ions and the associated anions, and can therefore not be reused or recycled without problems. Furthermore, the proportion of the vanadium in the calcium hydroxyvanadate which is dissolved by means of sodium carbonate is less than 50% and therefore completely unsatisfactory unless economically nonviable sodium carbonate excesses are employed.

[0012] The obvious routes or those already tried are therefore not suitable for dissolving vanadium from the calcium vanadate precipitate from the sodium chromate production process in such a way that an introduced, readily utilizable and commercial vanadium chemical is obtained and the valuable chromium present is obtained as a usable or recyclable product according to the above-described requirement.

SUMMARY OF THE INVENTION

[0013] The present invention now provides a route by means of which the vanadium present in the leaching solution from the fusion of chromium ore with alkali can be obtained as valuable vanadium oxide V₂O₅ and the valuable constituents chromium in the oxidation state +6 and sodium are made available as utilizable and recyclable chemicals.

[0014] The present invention therefore relates to a process for recovering the vanadium present in the chromium ore chromite as vanadium pentoxide, characterized by

[0015] 1. precipitation of a calcium vanadate precipitate from the sodium chromate solution (after pH-controlled removal of the residue from the chromium ore) by addition of calcium oxide, calcium hydroxide, calcium dichromate or calcium chromate at pH 12-13 set by addition of sodium hydroxide to the sodium chromate solution,

[0016] 2. separation of the calcium vanadate precipitate from the solution by customary solid/liquid separation operations, e.g. by filtration or centrifugation, and, if appropriate, washing of the precipitate with water or with water which has been made alkaline by means of dilute sodium hydroxide solution,

[0017] 3. treatment of the calcium vanadate precipitate with from 1.5 to 10 times its amount of water (ratio based on the dry weight of the residue), preferably from 2 to 4 times its amount of water, and (a) at least that amount of sodium carbonate which is stoichiometrically equivalent to the calcium content of the precipitate or an excess of sodium carbonate and the amount of carbon dioxide or sodium bicarbonate necessary to set the pH to 8.5-12.3, preferably 9-11, or (b) with a molar amount of sodium bicarbonate which is at least 1.0 times the molar amount of calcium in the precipitate (stoichiometry based on the CO₂ content) or an up to 3-fold molar excess of sodium bicarbonate, preferably from 1.3- to 2-fold amount of sodium bicarbonate, or (c) with carbon dioxide in an at least stoichiometric amount based on the calcium content and sodium hydroxide in the amount necessary to set a pH of at least 8, with this treatment preferably being carried out at an elevated temperature in the treatment solution, e.g. from 50 to 110° C., for a time of from about 0.1 to 5 hours, preferably from 0.5 to 1.5 hours,

[0018] 4. separation of the calcium carbonate formed from the solution by means of customary solid/liquid separation operations and washing of the calcium carbonate with water,

[0019] 5. electrolysis of the aqueous solution obtained after step 2 or 4 at 20-50° C. by introdu into the anode chamber of an electrolysis cell which is divided by means of a diaphragm or preferably by means of a cation-exchange membrane into an anode chamber and a cathode chamber, with the cathode chamber being supplied with an alkalizable aqueous solution, preferably a solution which can be introduced into and used in the process for producing sodium chromate; the pH of the solution in the anode chamber being generated to and maintained at a pH of from 1 to 4,

[0020] 6. withdrawing the solution from the anode chamber and heating it to a temperature above 80° C. to precipitate vanadium pentoxide and,

[0021] 7. separation of the vanadium pentoxide precipitated in step 6 from the solution by means of customary solid/liquid separation operations, washing of the V₂O₅ with water and drying of the V₂O₅.

DETAILED DESCRIPTION OF THE INVENTION

[0022] As an optional further step, part of the carbonate or bicarbonate which has been used in excess can be recovered as sodium bicarbonate after step 4 and before step 5 by passing carbon dioxide into the solution at a pH of about 7 and cooling the solution to from about −10 to 20° C., preferably from 0 to 5° C., and separation of the precipitated sodium bicarbonate from the solution.

[0023] It is of course also possible to accelerate the precipitation of the vanadium(V) oxide from the acidic anode chamber solution (anolyte) in step 7 by addition, after the electrolysis, of acidic agents which do not interfere, e.g. chromic acid or sodium polychromate, or solutions in which these are present.

[0024] The alkalizable aqueous solutions to be introduced into the cathode chamber in step 6 are particularly preferably water or sodium chromate, sodium dichromate, sodium polychromate, chromic acid, sodium hydroxide, sodium carbonate, sodium bicarbonate solution. This resulting cathode chamber solution is taken off in alkalized, sodium-enriched form.

[0025] The pH in the cathode compartment can be set within a wide pH range, preferably from 6 to 14, particularly preferably from 7 to 9.

[0026] The calcium carbonate obtained in step 4 can be passed to calcium oxide production for use in step 1, or can be converted into calcium chromate or calcium dichromate or calcium polychromate solution by reaction with sodium dichromate or sodium dichromate/sodium polychromate solution or sodium polychromate/chromic acid solution, or, if desired, can be deposited in a landfill as a nonhazardous, insoluble waste material or be used as an auxiliary in steel production. All other streams obtained can be fed back into the sodium chromate production process directly and without further treatment. The washing water which may be obtained in step 2 together with the filtrate generated previously forms the sodium chromate stream for further conversion into sodium dichromate. The sodium bicarbonate produced in the optional further step discussed above is added to the sodium bicarbonate which is formed in large quantities in the acidification step in which sodium chromate is converted into sodium dichromate by means of carbon dioxide under pressure and the combined sodium bicarbonate is passed to sodium carbonate production for the chromite fusion. The filtrate obtained in step 7 and the washing water contain sodium dichromate, sodium polychromate and/or chromic acid in amounts corresponding to the ratio applicable to the selected pH and are passed as acidification agent to the leaching of the furnace clinker with complete recovery of the chromium content.

[0027] After step 5, the solution can be taken from the cathode chamber and introduced into the sodium chromate stream of the process for producing sodium chromate, preferably directly into the leaching of the furnace clinker.

[0028] The entire process and also the individual steps can be carried out either batchwise or continuously.

[0029] The invention is illustrated by the following examples:

EXAMPLE 1

[0030] About 50 kg of the calcium vanadate precipitate (more accurately: calcium hydroxyvanadate) precipitated from sodium chromate solution at pH 12.5 (setting of pH by means of sodium hydroxide, addition of Ca as CaO) is dried at 110° C. for 5 hours and is then found to have the following analytical composition: 10.2% V 18.2% V₂O₅  7.3% Cr 14.0% CrO₃ 29.1% Ca 40.7% CaO  5.2% Na  7.0% NaO  1.3% Co₃ ²⁻

[0031] Main constituents are therefore about 38% of calcium vanadate Ca₅(VO₄)₃OH, about 10% of calcium chromate CaCrO₄, about 18% of CaO and about 13% of sodium chromate Na₂CrO₄.

EXAMPLE 2

[0032] 200 g of the dry calcium vanadate precipitate prepared in Example 1 is in each case mixed with 400 g of water and 155 g of sodium carbonate. The mixture was heated to 60° C. while stirring. To establish the desired pH values, carbon dioxide from a pressure bottle was introduced. The reaction mixture was stirred at 90° C. for 2 hours while continuing to monitor and adjust the pH. The mixture was then filtered and the filtrate was analyzed for chromium(VI) and vanadium(V). The filter residue was stirred twice in succession with 250 ml in each case of water at 60° C. for 10 minutes and then filtered off again. The washing water obtained as filtrate was likewise analyzed for chromium(VI) and vanadium(V). The filter residue which remained was dried at 110° C.

[0033] The results of the experiments carried out at pH 9, pH 10 and pH 111 are shown in the following table.

EXAMPLES 2a, 2b, 2c

[0034] V yield Cr yield [1%] [%] Washing Washing Amount of Example pH Filtrate water Total Filtrate water Total residue [g] 2 a 9 63.8 27.9 91.7 65.7 27.8 93.5 196 2 b 10 79 16.8 95.8 74.0 18.1 92.4 169 2 c 11 77.9 19.3 97.2 77.4 18.5 95.9 164

EXAMPLE 3

[0035] 5 kg of the dry calcium vanadate precipitate prepared in Example 1 are mixed with 11 l of water, 3700 g of sodium bicarbonate and 1000 g of sodium carbonate and heated to 90° C. while stirring vigorously. After one hour at this temperature, the calcium carbonate precipitate formed is filtered off and the filtrate is isolated. The precipitate is stirred twice in succession with 6.5 l each time of water at 90° C. for minutes and then filtered off again. The washing water obtained is isolated.

[0036] The filtrate from the calcium carbonate precipitation is transferred to a closable stainless steel vessel and in this vessel is cooled and at the same time treated with carbon dioxide gas. After displacement of the air by the carbon dioxide gas, the stainless steel vessel is closed and further carbon dioxide gas is introduced in the amount necessary to maintain a pressure of 3 bar. After a temperature of −5° C. has been reached, the mixture is stirred for another 30 minutes, the stirrer is then turned off and the contents of the stainless steel vessel are then pushed out through a filter by means of the carbon dioxide atmosphere.

[0037] The sodium bicarbonate which has been filtered off is dried at 110° C. and then weighs 1990 g as sodium carbonate.

[0038] The filtrate from the sodium bicarbonate precipitation is combined with the washing water from the calcium carbonate precipitation, and the liquid mixture obtained serves as starting material in some subsequent experiments. The vanadium(V) content was determined as 19.3 g of V/litre, and the chromium(V) content was determined as 13.3 g of Cr/litre.

EXAMPLE 4

[0039] The electrolysis cell used here comprises an anode chamber of titanium and a cathode chamber of stainless steel. As membrane, use was made of a Nafion 324® cation-exchanger membrane from DuPont. The cathodes comprise stainless steel and the anodes comprise expanded titanium metal coated with an electrocatalytically active layer of tantalum oxide and iridium oxide. Such anodes are described, for example, in U.S. Pat. No. 3,878,083.

[0040] The distance between the electrodes and the membrane was in all cases 1.5 mm.

[0041] An aqueous solution containing 361 g/l of sodium dichromate Na₂Cr₂O₇.2H₂O and 57 g/l of sodium chromate Na₂CrO₄ was introduced into the cathode chamber. The introduction of this solution was controlled so that a pH of 7-8 is maintained in the outflow from the cathode chamber.

[0042] A solution as obtained in Example 3, containing 19.3 g of V/l and 13.3 g of Cr, is fed into the anode chamber. The introduction of this solution is set so that the pH of the outflowing solution is from 3.4 to 3.7.

[0043] The current was 23 A, so that the current density based on the area of 11.4 cm×6.7 cm facing the membrane was 3.1 kA/m². The electrolysis temperature is maintained at 35° C. by means of cooling.

[0044] The solution which flowed from the anode section is heated to 90° C. and kept at this temperature for 1 hour. The precipitated vanadium pentoxide remains together with the supernatant solution during the cooling phase and is then filtered off, washed with a little cold water and dried.

[0045] 42.83 g of dry V₂O₅ having a vanadium content of 53.5% by weight are obtained from 1.51 of this stream flowing from the anode compartment, corresponding to a yield of 79% of vanadium which was present in the solution.

EXAMPLE 5

[0046] The experimental set-up of Example 4 and the introduction of reagents into the electrolysis cell as described in Example 4 are employed. The current density and the temperature are retained, but, in contrast to Example 4, the pH of the sodium chromate solution flowing out from the cathode chamber is set to 12-12.5.

[0047] The vanadium/dichromate solution flowing out from the anode compartment at a pH of 3.5 is brought to a pH of 1.5 by means of chromic acid and is then heated to 95-100° C. The precipitation of vanadium pentoxide occurs significantly more quickly than in Example 4, and the precipitated V₂O₅ is filtered off after half an hour. 

We claim:
 1. Process for recovering the vanadium present in the chromium ore chromite, as vanadium pentoxide, as a by-product from the vanadium-containing sodium chromate solution produced in the process of preparing sodium chromate and sodium dichromate from said ore, which comprises:
 1. precipitating calcium vanadate from the sodium chromate solution by adding calcium oxide or calcium hydroxide or calcium dichromate or calcium chromate at a pH of 12-13, said pH being established by addition of sodium hydroxide to the sodium chromate solution to give a low-vanadium sodium chromate solution,
 2. separating the calcium vanadate precipitate from the solution to form a low vanadium content sodium chromate solution,
 3. treating the calcium vanadate precipitate with from 1.5 to 10 times its amount of water, based on the dry weight of the residue precipitate), and (a) at least an amount of sodium carbonate which is stoichiometrically equivalent to the calcium content of the precipitate or an excess of sodium carbonate and an amount of carbon dioxide or sodium bicarbonate sufficient to establish a pH to 8.5-12.3, preferably 9-11, or (b) with a molar amount of sodium bicarbonate which is at least 1.0 times the molar amount of calcium in the precipitate, stoichiometry based on the CO₂ content, or an up to 3-fold molar excess of sodium bicarbonate, or (c) with carbon dioxide in an at least stoichiometric amount based on the calcium content and sodium hydroxide in an amount sufficient to establish a pH of at least 8, to form a calcium carbonate precipitate,
 4. separating the calcium carbonate precipitate from the solution,
 5. electrolysis of the aqueous solution obtained after step 2 or 4 at 20-50° C. by introducing the solution into the anode chamber of an electrolysis cell which is divided by a diaphragm into an anode chamber and a cathode chamber; the cathode chamber being supplied with an alkalizable aqueous solution, and generating and maintaining a pH of from 1 to 4 within the anode chamber,
 6. withdrawing the solution from the anode chamber heating it to a temperature above 80° C. to precipitate vanadium pentoxide and,
 7. separating the vanadium pentoxide precipitated in step 6 from the solution and washing the separated vanadium pentoxide with water.
 2. Process according to claim 1, wherein any excess carbonate or bicarbonate which has been used in step 3 is recovered as sodium bicarbonate after step 4 and before step 5 by passing carbon dioxide into the solution at a pH of from 7 to 5 and cooling the solution to from about −10 to 20° C. and separating the precipitated sodium bicarbonate from the solution.
 3. Process according to claim 1, the alkalizable aqueous solution introduced into the cathode chamber in step 5 is an aqueous solution of sodium chromate, or sodium dichromate, or a mixturethereof.
 4. Process according to claim 1, wherein the remaining after separation of the vanadium pentoxide in step 7 is used as pH-regulating agent and as partial replacement for sodium dichromate solution in the furnace clinker dissolution process of the process for producing sodium chromate.
 5. Process according to claim 1, wherein the solution in the cathode chamber in step 5 is withdrawn and is returned to the process for producing sodium chromate.
 6. Process according to claim 1, wherein the separation step 2 is carried out by filtration or centrifugation.
 7. Process according to claim 1, wherein the precipitate separated from the solution in step 2 is washed with water or with water which has been made alkaline by means of sodium hydroxide.
 8. Process according to claim 1, wherein, in step 3, the calcium vanadate precipitate is treated with from 2 to 4 times its amount of water.
 9. Process according to claim 1, wherein variant c) of step 3 is carried out at a temperature of 50-110° C.
 10. Process according to claim 1, wherein the treatment with the 1.3- to 2-fold amount of sodium hydrogen carbonate in variant (a) in step 3 and the treatment in variant (c) are carried out for a time of from about 0.1 to 5 hours.
 11. Process according to claim 9, wherein variant c) of step 3 is carried out for a time of from about 0.1 to 5 hours. 